1. Field of the Invention
The invention relates to an erasable magneto-optical recording medium comprising a substrate on one side of which there is provided a first magneto-optical recording layer having a first Curie temperature and a second magneto-optical recording layer having a second Curie temperature, such layers having an easy axis of magnetization perpendicular to the plane of the layer, superposed information bits in the form of magnetic domains being inscribable in both layers by a combination of irradiation with laser light and a modulated magnetic field.
The invention also relates to a method of simultaneously erasing existing information and inscribing new information in such a magneto-optical recording medium.
2. Description of the Related Art
In a MO (=magneto-optical) recording medium, data are represented by magnetized domains or bits. The direction of magnetization of the domains extends perpendicularly to the plane of the MO-recording layer. The bits can be detected by means of the MO-effect, which causes the direction of polarization of polarized light to change as a function of the direction of magnetization of the domains when such light is reflected from (Kerr-effect) or passed through (Faraday-effect) the layer.
The magnetized domains are obtained by locally heating the MO-recording layer by means of a focused laser light beam to a temperature above the Curie temperature of the material of the MO-recording layer. During cooling to a temperature below the Curie temperature, the magnetization is locally oriented according to the direction of an externally applied magnetic field and then becomes frozen-in. This method is also termed thermomagnetic recording. A customary manner of inscribing information in a MO-recording layer consists in rotating a disc-shaped recording medium while a permanent external magnetic field is applied. A laser light beam which is modulated according to the information to be recorded inscribe inscribes magnetic domains in the MO-recording layer. A disadvantage of said laser-modulation method is that the existing information cannot be directly overwritten with new information. The existing information must first be erased, by using a permanent magnetic field and a continuous laser light beam. A suitable method of directly overwriting existing information with new information (termed direct overwrite) is magnetic field modulation (MFM). In MFM it is the external magnetic field which is modulated according to the information to be recorded, the focused laser light beam having a constant power. The external magnetic field is generated by a magnet coil.
In the case of MO-recording using MFM, a MO-recording layer is desired whose direction of magnetization can change at weak magnetic fields, because this enables the dissipation of the magnet coil to be reduced and/or the distance of the magnet coil to the MO-recording layer to be enlarged. Technically, it is very difficult to generate a strong magnetic field having a high frequency (in the order of magnitude of 10 MHz).
In European Patent Application EP-A-225141 a description is given of such a magneto-optical recording medium which is suitable for direct overwriting. In said Application, use is made of two "exchange"-coupled MO-layers which have an opposite magnetization at temperatures between room temperature and the Curie temperature, so that the opposing demagnetizing fields are small and the MO-layer can be inscribed with a smaller external magnetic field. In this known MO-recording medium, both MO-layers have an opposite direction of magnetization. Also the superposed domains have opposite directions of magnetization.
A disadvantage of the known MO-recording medium resides in the fact that the technology used for the manufacture of the exchange-coupled MO-layers is difficult to control. Said MO-layers are provided on a substrate by means of vacuum evaporation or sputtering. The exchange-coupling acts over only a few interatomic distances, so that any contamination or oxidation of the first MO-layer substantially reduces the exchange-coupling with the second MO-layer.